A serious environmental problem facing the world today is global climate change, i.e., global warming, which has been linked to the increased production of greenhouse gases, namely, carbon dioxide. Growing evidence details the accumulation of greenhouse gases in the air, the most important of which is carbon dioxide, as having an associated role in causing global climate warming. Since 2001, carbon dioxide has accounted for over 82% of all greenhouse gas emissions in the United States. Nearly 60% of carbon dioxide is emitted by utility or industrial power generation plants, which rely on fossil fuel combustion. The continuing increase in the greenhouse gas carbon dioxide in the atmosphere highlights the need to develop cost-effective, reliable and safe methods of carbon dioxide capture from flue gases, among other sources.
More specifically, in order for carbon-rich fossil fuels, such as coal and natural gas, to remain viable and environmentally acceptable energy sources throughout the 21st century and beyond, new technologies that allow capture and sequestration, utilization, or recycling of carbon dioxide need to be developed at reasonable costs. The sequestration of carbon dioxide would allow the use of carbon-based fuels to meet the world's increased energy demands far into the future, without further increasing the atmospheric concentration of carbon dioxide. Additionally, for fossil fuels to maintain their predominance in the global energy market, the disposal of carbon dioxide and the elimination of carbon dioxide emissions to the air are ultimate goals for curbing the problem of global warming.
Environmental pollution stemming from fossil-fueled power plants is of particular concern. Power plants emit greenhouse gases, notably carbon dioxide. Although certain methods and technologies are being developed that reduce carbon dioxide emissions, they are often expensive and require considerable energy. This carbon dioxide can be “captured” or removed from the flue gas using several known methods including air separation/flue gas recycling, amine scrubbing, cryogenic fractionation, and membrane separation, discussed further below.
Purified carbon dioxide is also a market product of significant value, such that a stream of purified carbon dioxide is a valuable product. It would be very useful to remove carbon dioxide from, for example, a flue gas stream so as to produce a supply of purified carbon dioxide while purifying the flue gas stream.
There are three direct techniques that are used primarily to reduce carbon dioxide emission from plants that burn fossil fuels such as coal, natural gas, and oil. These techniques are known as pre-combustion capture, oxy-fuel combustion capture, and post-combustion capture.
Pre-combustion capture involves “integrated gasification combined cycle” (IGCC) technology. This is an upstream process for gasifying coal in combined cycle power plants (gas and steam power plants.) This process first converts coal to a synthesis gas (syngas) inside a gasifier at high temperatures and under pressure. The syngas mainly comprises hydrogen and carbon monoxide. Next a water-gas shift reaction is then used to create carbon dioxide and additional hydrogen. The carbon dioxide is then scrubbed, and the hydrogen is burned to produce power, e.g., in a gas turbine. The advantage of this approach is that it is much less expensive than the post-combustion capture process. The disadvantages are that there are only a few IGCC plants in the existing coal fleet and IGCC plants are more expensive than post-combustion (PC) plants (discussed below) when costs of carbon dioxide capture for the latter are not included.
Another carbon capture technique is known as “oxy-fuel combustion”. In this process coal is combusted in an atmosphere consisting of pure oxygen and carbon dioxide. This means that the resultant flue gas is not diluted by nitrogen from the air, but primarily consists of carbon dioxide and water vapor. The water vapor is easily condensable, and leaves behind a highly concentrated carbon dioxide stream. The carbon dioxide can then be compressed and transferred to storage. Disadvantages include the requirement of a source of oxygen.
The third principal carbon capture technique is “post-combustion capture.” This technique uses amines to separate the carbon dioxide in conventional coal-fired power plants following desulphurization of the flue gas. This is the only method for retrofitting existing power plants. However there are several concerns regarding this technique such as high capital cost with estimated 75% increase from adding carbon dioxide capture, large footprint required for carbon dioxide capture equipment, numerous operational concerns, and high energy penalty for carbon dioxide stripping and regeneration of solvent.
In a more specific discussion of known post-combustion methods for purification of flue gases, these generally involve several separate treatments to provide carbon dioxide having high purity. These purification treatments including washing, absorption, adsorption, desorption, and the removal of reducing substances. Washing generally involves a water absorption shower (water wash) to remove solids and at the same time to cool the combustion gases. Various scrubbing solutions are generally employed to remove contaminants and to reduce the components in the combustion gas mixture to carbon dioxide.
The washed and scrubbed flue gases are then separated to obtain carbon dioxide rich fraction. In one separation method, the combustion gas mixture is circulated through a counter-current shower of an absorbing solution. Carbon dioxide can be desorbed by heating the carbon dioxide saturated solution.
The purified and separated carbon dioxide is then compressed to a pressure in the range from about 230 psia to about 400 psia, dried by contacting the gas with a regenerable desiccant, and liquified by lowering the temperature of the gas. Finally, a rectification distillation step eliminates the small amount of nitrogen, oxygen, and argon to provide carbon dioxide having a high purity.
As described above, the prior art in this field continues to suffer from overall efficiency of the process and undesirable costs and expenses. These problems have been significantly mitigated by the system and process of this invention for capturing and releasing carbon dioxide.